Molybdenum-base alloy



Sept. 966 w. H. CHANG 3,275,434

MOLYBDENUM-BASE ALLOY Filed April 13, 1964 EELHT/ VE fiBUNDQNCE l l l lI I l l 2000 2250 2500 2750 3000 3250 3500 3750 4000 4200 TEMPE/QATUEE,"F

His A t rzvnek United States Patent 3,275,434 MOLYBDENUM-BASE ALLOYWinston H. Chang, Cincinnati, Ohio, assignor to General ElectricCompany, a corporation of New York Filed Apr. 13, 1964, Ser. No. 359,0805 Claims. (Cl. 75--176) .This is a continuation-in-part of applicationSer. No. 227,046, Chang, filed Sept. 28, 1962, now abandoned, andassigned to the assignee of the present invention.

This invention relates to molybdenum alloys and, more particularly, tomolybdenum alloys having a combination of high temperature strength andlow temperature duetility as a result of the alloying of molybdenum withjudiciously selected amounts and proportions of the elements, titanium,zirconium and carbon.

Some of the advantages of alloying molybdenum with titanium andmolybdenum with zirconium have been set forth in United States Patents2,678,269 and 2,678,271, respectively. Furthermore, these two patentsmention that carbon in very small quantities can be present. In additionwith regard to carbon, United States Patent 2,947,624 indicates thatcertain advantages can be obtained from the use of carbon in molybdenumalloys in the range of 0.30 weight percent or higher.

In the modern design of molybdenum alloys the mechanisms of solidsolution alloying and precipitation alloying have been extensivelyinvestigated. One of the molybdenum alloying systems on which much workhas been done contains a minor but significant amount of titanium, asomewhat smaller amount of zirconium and some carbon. Severalmodifications in this alloy system have been investigated and some arepresently being commercially exploited. However, in my persentinvestigations concerning the phase relationships and metallurgy ofmolybdenum alloys which can be precipitation hardened by combination oftitanium-, zirconiumand molybdenum-carbides, I have found that suchlevels of titanium as have been advocated in the prior art aredeleterious to both strength and ductility. Relatively small amounts ofzirconium stabilize the precipitated titanium carbides and also formmore stable complex carbides of zirconium and titanium. By appropriatesmall but critical adjustments of the levels of titanium, zirconium andcarbon and the ratios of zirconium plus titanium to carbon, improvementsover similar prior art molybdenum alloys have been obtained. However,many of the improvements have been in the direction of higher strengthat elevated temperatures accompanied by lowered workability andductility at room temperature and below.

Furthermore, many of the previously known alloys in themolybdenum-titanium-zirconium-carbon system contain significant amountsof MOZC. Interactions between Mo C and other precipitating phases aregenerally undesirable. Massive deposits of Mo C often are formed as amolten ingot freezes, and the carbon is not readily redistributedthrough the martix on decomposition of the *Mo C to other precipitatesduring aging. Also, unstable lower temperature precipitates that convertto Mo C at moderate temperatures do not provide the desired strengthlevels. Preferential distribution of Mo C in the grain boundaries isquite deleterious to properties. Thus, molybdenum alloys that can behardened by precipitation from solution in the absence ofditficult-to-control interactions between different precipitated phasesare greatly to be desired, and are not generally found among previouslyknown alloys.

It is the desire of development metallurgists to produce alloys ofunusual strength at high temperatures. However, it is important to thedesigners of highly stressed articles, such as turbine buckets for gasturbine engines, that an alloy b'e ductile at lower temperatures toavoid brittle fractures during the starting of such apparatus.Furthermore, the designers of articles for use in flight apparatusprefer materials which have a high strength-to-density ratio so thatlighter weight parts can be fabricated.

It is a principal object of the present invention to provide an improvedmolybdenum-base alloy having unusual high temperature strength up toabout 3500 F. along with good room temperature ductility.

It is another object of this invention to provide a high strengthmolybdenum-base alloy having a high strengthto-density ratio.

Another object of the invention is to provide molybdenum-base alloyswith optimum contents and proportions of titanium, zirconium and carbonsuch that they can be treated to develop desired properties at elevatedtemperatures and at room temperature and below.

Another object of the invention is to provide a precipitation-hardenablemolybdenum-base alloy in which the formation of undesirable Mo C isavoided.

Still another object of the invention is to provide aprecipitation-hardenable molybdenum-base alloy which is notdeleteriously susceptible to aging during use at elevated temperatures.

These and other objects and advantages will be more readily recognizedfrom the following detailed descrip tion, tables and examples which arerepresentative of but are not intended to be limitations on the scope ofthe present invention.

The drawing is a graphic representation of the interrelations of carbideprecipitates in various molybdenumbase alloys including the alloys ofthe present invention.

It has been found that molybdenum-base alloys containing judiciouslyselected concentrations and proportions of titanium, zirconium andcarbon have properties unexpected from that which was previously known.The alloys of the present invention consist essentially of about, byweight, 1.25-2% Ti, 0.40.7% Zr, ODS-0.2% C, the atomic ratio of(Ti+Zr)/C being from about 2:1 to about 6:1, with the balance Mo.Percentages givenin this specification are by weight except whereotherwise indicated.

In titanium and carbon-bearing dispersion-hardened molybdenum-basealloys, the major dispersion phase is titanium carbide. However, anexcess of carbon with regard to the amount of titanium will allow theformation of massive Mo C which is not efiect-ive for strengthening inthe manner of the finely dispersed titanium carbide. It has beenrecognized that a careful and judicious selection of .amounts andproportions of the elements titanium, zirconium and carbon results inthe formation of a (Ti, Zr) complex carbide.

The inclusion of too small an amount of titanium will not provide enoughtitanium carbide for strengthening and also will permit the formation ofMo C. Too high a titanium content will lower the alloys melting point,form massive carbides, and increase the diffusion rate to the detrimentof high temperature strength.

Zirconium in relatively small quantities stabilizes the titanium carbidephase. But zirconium within the range of the present invention forms acomplex zirconiumtitanium carbide which is more stable than titaniumcarbide.

The function of carbon in the alloy of the present invention is that itis necessary for carbide dispersion. A critical amount of carbon must bepresent with the range of titanium and zirconium in order to form a linedispersion of the complex titanium-zirconium carbide. If there is toolarge an amount of carbon or two low an amount of titanium andzirconium, too much Mo C forms. This molybdenum carbide does not providethe strengthening effect of the complex titanium-zirconium carbide.

'Even though the .titanium-zirconium-carbide dispersoid 1 is much harderthan the molybdenum matrix, it also is desirable that the precipitatedparticles be of a size small enough that there be few if anydislocations in each par-;

ticle, and so that dislocation movement in the particle is very limitedif existent at all, thereby greatly increasing the etfective hardnessofthe dispersed particles. A

finer particle size also leads to smaller interparticle spacing, therebyimproving the mechanical properties of the V alloys. "'Relative'totitanium-zirconium complex carbide precipitates in alloys of theinvention, molybdenum carbide.

generally precipitates in the form of massive, sometimes featheryparticles which serve little useful purpose in strengthening the alloyor in making it ductile. Although molybdenum carbide, serves a purposein some related alloys as an intermediate phase for the purpose ofpromoting con-trolled aging reactions, it is not generally desirable asa constituent of a final alloy in condition for applica-- tion.Moreover, residual molybdenum carbide left after 1 controlled heattreatment in processing can cause. deleterious on-the-j-ob aging andembrittling of such alloys by conversion to other carbides at elevatedtempera-.

ture during application, particularly with the aid of straininducedaging. Although for some applications it may be desirable to have analloy that will age during use, it is also quite desirable to havealloys that will not age during use for other applications.

4 utilized in the formation of useful and controllable precipitates, andin which unwanted aging reactions can be avoided.

The following Table I is representative of the number of the alloyswhich were considered in the study ofthe alloy of the present invention.

Alloys 1, 2,1 3A and 4 were made in 251 pound heats by melting in avacuum arc furnace; The composition given for alloyTZC is one of severalwith which the name TZC has beenused in the art. Alloys 1, 2, 3Aand '4were then extruded and swaged to a total of, 93% reduction in area to A"bars. These bars weretested for strength and ductility after beingstress relieved $2100";

F.fOI' about one hour. The results of these tests,,which were conductedin a vacuum at the temperatures shown,

are given in Table 11.

TABLE IL-TENSILE TEST RESULTS ON SWAGED ALLOYS Temp. U.T.S., 0.2% Y.S.,El. (percent RA Strength to Alloy F.) ksi. ksi. in 1) (percent) DensityRatio (in.)

avoided. The precipitated carbon is present in the form of (Ti, Zr)C. Bythe choice ofappropriate combinations of composition and thermal andmechanical treatments,

alloy bodies can be produced within the limits of the pres cnt inventionin which the carbide present is efliciently .75 tures from about roomtemperature (78.F.) to 3500 F5 In Table 11 and subsequent tables, U.T.S.means:

Reduction in Area and 'ksi. means ,thousands of pounds per square incThe densities .of alloys 1, 2,,

3A,.3B and 4 were respectively, in pounds per square inch: 0.359; 0.360; 0.360; and 3.342; 7

By comparison, the strength of alloy 3A at temperais unusually high andunexpectedly better than other alloys tested.

Alloy 3B was produced by consumable arc melting in a vacuum furnace to a31-pound ingot. The ingot was extruded at 3500 F. to sheet bar at anextrusion ratio of 4 to 1. The resulting bar, 1.93 inches wide by 0.94inch thick, was bisected along the plane of thickness and rolled to 0.05inch thick sheet. The rolling procedure consisted of 50% reduction inthickness at 2750 F. at 10% per pass, 50% at 2500 F. at 10% .per pass,and final reduction to 0.05 inch thick at 2200 F. at 10% per pass. Theheating was done in a hydrogen furnace, with the material being soakedat temperature for 5 to 10 minutes between passes. Highquality sheet wasproduced with a material recovery of greater than 90%. The resultsindicated that the alloy is quite fabricable. With appropriateadjustments in techniques, the melting difliculties should be readilyovercome.

Bend tests were preformed on the sheet at room temperature in both thestress-relieved condition (2100 F./ 1 hour) and the recrystallizedcondition (3000 F./1 hour). In the stressed-relieved condition the alloysustained a full bend around less than a IT radius (T equals thicknessof the sheet), while the recrystallization treatment only raised theradius for full bend to 2T, indicating no significant embrittlement onrecrystallization.

Tensile tests were performed on 0.05 inch thick sheet of alloy 3B asstress-relieved at 2200 F. for onehour and as-recrystallized at 3200 F.for one hour. The results are presented in Table HI below. SR indicatesthe stress-relieved condition and RX indicates the recrystallizedcondition. The specimens had 0.5 inch long gauge lengths, inch wide. Themajor surfaces were as-pickled in the rolled condition, and the testswere performed on an Instron machine at a nominal strain rate of 0.05per minute. Elevated temperature testing was done in a vacuum of about10* torr.

TABLE III.'IENSILE PROPERTIES OF SHEET ALLOY 313 Test Condi- TensileTensilefDen- 0.2% Yield Elonga- Texnp., tion Strength, sity Ratio,Strength, tion,

F. ksi. iu. 1[)- ksi. Percent The tensile data on the alloy 3B sheetcompare very favorably with those on the alloy 3A swaged material shownin Table II for the following reasons:

First, as in the swaged condition of alloy 3A, the 0.05-in. sheetmaterial possesses an excellent combination of high-temperature strengthand low-temperature ductility which appears far superior to that of anyother molybdenum sheet alloy reported in the literature. Thehigh-temperature strength is illustrated by the tensile strength of28,600 and 14,300 p.s.i. at 3000 and 3500 F., respectively,corresponding to a tensile strength to density ratio of 80,000 and40,000 inches at the respective temperatures. This remarkablehigh-temperature strength is accompanied by room-temperature tensileelongation of almost 20% (probably corresponds to a 10-15% elongation ina l-in. gage length) which agrees well with the bend test results.Second, the literature is full of examples which show much lowerstrength in sheet form than in bar or plate form. The sheet alloy 3Bmaterial is shown to have strength and ductility properties very similarto those of the strongest swaged alloy 3A condition up to 3000 F. Abovethe latter temperature, the swaged material becomes superior mainlybecause of its higher recrystallization temperature of 3500 F., ascompared with 3000" F. or 3200 F. for the sheet material. In thisconnection, it may be noted that the titanium and carbon contents inalloy 3B are 1.28% and 0.08%, respectively, which are lower than the1.6% and 0.13% present in alloy 3A. The excellent properties of thesheet material, in spite of the possibly adverse chemical difierences,further accentuate the importance of processing and heat treatment incontrolling microstructure and properties.

Table IV compares alloys 3A and 3B with some othertitanium-zirconium-carbon-molybdenum alloys. The data on alloy 3B arefrom sheet specimens, those on the other alloys are from swagedspecimens.

TABLE IV Strength to Alloy Temp. U.T.S., ksi. E1. percent Density F.) in1 inch Ratio The data of Tables II and HI show that alloys of theinvention have an unexpected and unusual combination of high strength atlow as well as at high temperatures along with good low temperatureductility not found in closely similar alloys. It has been found that apreferred range in which alloys of the invention are included consistsessentially of, by weight, 1.252% titanium, 0.4-0.7% zirconium,0.080.-2% carbon, with the atomic ratio of (Ti+Zr)/C being from about3:1 to about 5:1, with the balance molybdenum. The higher titaniumbearing alloys as represented by alloys 4, 45, 6, 44'and 1720 were verymuch weaker and had a much lower strength-to-density ratio than do thealloys of the present invention. This is true even for alloy 44 whichhas the same general zirconium and carbon content as the alloy of thepresent invention but with a higher titanium content. Referring to TableIV, for example, it is seen that alloy 44 has less than half thestrength of alloy 3A at 3000 F. with a change of less than about 1.5%titanium.

Alloy TZC has too low a content of alloying additions and too low aratio of titanium plus zirconium to carbon to possess the desiredproperties. Specifically, the ratio leads to the formation of Mo C whichis difficult to control and the undesirable characteristics of whichhave been discussed above. The higher titanium content alloys withoutzirconium, alloys 4 and 1720, had such high ratios of titanium to carbonthat it was quite difiicult to produce a fine dispersion of precipitate.These alloys have relatively low recrystallization temperatures andunattractive strength properties.

Similarly, alloys 4 and 45 having lower carbon and alloy 6 having highercarbon content are substantially weaker than the alloy of the presentinvention. Alloys 1 and 4 show dramatically the effect of the absence ofzirconium in the presence of titanium and carbon, and in particular withregard to alloy 4, the effect of high titanium and low carbon in theabsence of zirconium.

Although the presently claimed compositions may appear superficially tobe similar to those of prior art, they are in fact critically differentas shown in part by a par-. ticular kind of susceptibility of alloys ofthe invention to alterations in their mechanical properties throughther- 7 mal and mechanical treatments. This susceptibility or capabilityis,in itself, a valuable property. It is present in alloys within thecomposition limits stated inthe specification and the claims, and havingthe specified ratios between the Group IV-A metals (zirconium andtitanium) and carbon. These ratios as claimed establish a criticalrelationship between the Group IV-A metals and carbon which determinesthe type and morphology of carbides which will be precipitated withvarious types of heat treatments. The claimed ratios are all above athreshold .ratio which is the maximum which would generally allowprecipitation of Mo C. Therefore, as stated above, all the carbidesprecipitated in alloys of the invention are carbides of the Group IV-Ametals. Previously known alloys in the molybdenum-titanium:zirconium-carbon system which do not meet the ratio or proportion limitsin addition to the composition limits produce alloys that are differentin kind from those of the present invention. zirconium plus titanium tocarbon ratio substantially less various heat treated and workedconditions.

For example, an alloy having a.

more, the present invention is directed toward alloys having a carboncontent low enough in relation to the zir conium and titanium contentsto substantially avoid .the

presence of Mo C, as distinguished from alloys that sists essentiallyof, by weight, 1.5-2% Ti, 0.4-0.7% 'Zr,

0.1-0.2 C with the balance .Mo. These composition limits, of course,establish a range of ratios of (Ti+Zr) /C. The two extreme limits ofthis range on an atomic basis are about 2.13:1 and about 5.94:1. 'Inlike manner, the

ratio of (Ti+Zr)/ C inherent in the composition of alloy 3A'is about3.70:1, and inalloy 3B about 5:1.

In order, to enable a better understanding of the criticality of theamounts and proportions of alloying additions in the alloys of theinvention, consideration will now be given to various types of carbideprecipitation reactions in molybdenum-base alloys containing variousamounts and proportions of titanium, zirconium and carbon. The drawingintegrates the information discussed below and should be referred to forperspective and for comparison of the characteristics of the variousalloys. The addition of titanium in Mo-C alloys first results in theappearance of TiC at low temperatures, with Mo C remaining as thehigh-temperature equilib rium carbide. .In the TZC alloy which had a (Ti +Zr)/ C ratio of 1.8, the stability ranges were characterized bycomplete TiC dissolution at 3500 F. and persistance of Mo C down toabout 3000 F. The effect of further increase in Ti is-seen to raise theequilibrium solution temperature of TiC, while restricting the stabilityrange of Mo C. Thus, in alloys 1 and 2 which contain 1.6- 1.8% titaniumwith (Ti+Zr)/C ratios of 3.2-3.5, the TiC phase did not completelydissolve until about 3750 F., while the Mo C was no longer stable below3300 F.

This effect of titanium continues until a concentration is reached whicheliminates the M0 0. phase completely, as in the case of alloys 4 and1720. It is interesting to note that while a substantial amount of M0 0remained stable in alloy 1 which had a Ti/C ratio of 3.5,none existed inalloy 1720 which had a Ti/C ratio of 5. The threshold Ti/C ratio foreliminating the Mo C phase thus appears to lie at about 4.5 in theMo-Ti-C system.

Compared with titanium, zirconium proved to be much more potent instabilizing the monocarbide at the expense of M0 0. The diiference,indeed, appears to be zirconium in alloy 3A was suflicient to eliminateMo C which was a predominant high-ternperature carbide in Furthermore,although the atomic concentra-' alloy 1. tion of titanium in alloy 3(about 3.2% was some five times that of zirconium, the monocarbide wasfound to. be zirconium-rich, as evidenced by the] large tlatticeparameter and the strong zirconium emission intensities: These resultsindicate that the threshold Zr/C ratio inthe Mo-Zr-C system isconsiderably below the Ti/C ratio in the Mo-Ti-C system. More recentdata indicate the threshold Zr/C ratio to be below 2.5,"as compared Ewith the Ti/C threshold ratio. of 4.5 in the Mo-Ti-C.

system.

In alloys of the invention, the zirconium limits claimed,

particularly the lower limit of 0.4%, are critical to the properties,structure, andcharacteristics of the alloys.

:Below 0.4%, and generally outside of the composition range of 0.40.7%,zirconium does not give the desired aging kinetics for the formation andgrowth of the com-( plex carbides, and does not give the desiredstability and morphology to the precipitates. Moreover, insuflicientamounts of zirconium will not insure against the presence of deleteriousamounts'of M0 0, the undesirable charac-' teristics of which have beendiscussed above.

As compared to. alloy 2, alloy 3A represents an interesting case byvirtue of its moderately higher (Ti+Zr) /C ratio and zirconium content.These two characteristics influence the carbide nucleationinsuch afavorable manner that the alloy develops a fine and extensive dispersionboth by partial precipitation on cooling and uponwork ing or aging:Thus, the carbide is utilized more eflectively in alloy 3A than inalloys 1 or 2,1 as it is not tied 1 up in large clusters of decomposedMoc sometimes seen in the worked or aged conditions of the latter twoalloys.

From lattice parameter studies, it is apparentthat essentially all thezirconium addition in alloy 3A is consumed in the .(Zr, Ti.)C phase,thereby indicating thatthe,

amount of zirconium dissolved in the molybdenum matrix is so small as tomake insignificant any possible soluof alloy 2 can be improved, thoughwitha slight sacrifice of strength at room temperature, through the useof -Heat Treatment B. However, through the use of such heat treatment,there is an improvement in high temperature strength as well, althoughalloy 3A within thepreferred range of this invention is unexpectedlyim-. proved with all heat treatments shown. V .Heat Treatments A, B andC, following extrusion at 3500-3650 F., are as follows:

Heat Treatment A: As-extruded material swaged 93% at 2500 F.-2100 F.;stress-relieved at .2100 F. for, one

hour in hydrogen- Heat Treatment B: As-extruded material aged at'2500" VF. for 50 hours before swaging 93% and stress-relieved as in HeatTreatment A."

Heat Treatment C: Assextruded material swaged 88%; at 2500 F.-2100 F.;stress-relieved at12200 F. for one hour in hydrogen.

As used in Table TABLE V Temp. Heat U.T.S. 0.2% Y.S. El. (percent RA.Strength to Alloy F.) Treatment (ksi.) (ksi.) in 1") (percent) DensityRatio (inches) One-hour vacuum annealing at various temperatures wasfound to lead to recrystallization in alloy 3B at temperatures between3000 and 3250 F. depending on variations in heat treatment before therolling process. Material which was given a vacuum anneal of 3200 F. forone hour prior to rolling recrystallized at about 3000 F after rolling,while the material that was rolled directly from the extruded conditionrecrystallized at about 3250 F. Although the recrystallized grain sizewas slightly larger in the directly rolled material, neither materialunderwent substantial grain growth in one hour at temperatures below3750 F.

The heat treatment and processing of the alloys of this inventioninvolve control of the interplay between dispersion hardening and strainhardening and can be applied to usefully vary the properties of alloysof the invention.

Thus, the present invention involves a dispersion-hardenedmolybdenum-base alloy of unusual characteristics in an unexpectedlycritical range of compositions and proportions in order to control theamount, type and distribution of complex carbides formed.

Although this invention has been described in connection with specificexamples, those skilled in the arts of metallurgy and heat treatmentwill recognize the variations and modifications of which the presentinvention is capable without varying from its scope.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An improved molybdenum-base alloy consisting essentially of about, byweight, 1.252% titanium, 0.4-

0.7% zirconium, ODS-0.2% carbon, the atomic ratio of titanium pluszirconium to carbon being from about 2:1 to about 6:1, with the balancemolybdenum.

2. An improved molybdenum-base alloy consisting essentially of about, byweight, 1.25-2% titanium, 0.4- 0.7% zirconium, ODS-0.2% carbon, theatomic ratio of titanium plus zirconium to carbon being from about 3:1to about 5:1, with the balance molybdenum.

3. An improved molybdenum-base alloy consisting essentially of about, byweight, 1.6% titanium, 0.6% zirconium, 0.13% carbon, the atomic ratio oftitanium plus zirconium to carbon being about 3.7: 1, with the balancemolybdenum.

4. An improved molybdenum-base alloy consisting essentially of about, byweight, 1.28% titanium, 0.58% zirconium, 0.08% carbon, the atomic ratioof titanium plus zirconium to carbon being about 5:1, with the balancemolybdenum.

5. The alloy of claim 1, wherein the precipitating phase is essentiallyall a complex carbide of zirconium and titanium.

References Cited by the Examiner V UNITED STATES PATENTS 2,678,2695/1954 Ham l76 2,678,271 5/1954 Ham 75176 2,947,624 8/1960 Semchyshen75-176 2,960,403 11/ 1960 Timmons 75176 DAVID L. RECK, Primary Examiner.W. o. TOWNSEND, Examiner.

1. AN IMPROVED MOLYBDENUM-BASE ALLOY CONSISTING ESSENTIALLY OF ABOUT, BYWEIGHT, 1.25-2% TITANIUM, 0.40.7% ZIRCONIUM, 0.05-0.2% CARBON, THEATOMIC RATIO OF TITANIUM PLUS ZIRCONIUM TO CARBON BEING FROM ABOUT 2:1TO ABOUT 6:1, WITH THE BALANCE MOLYBDENUM.